CI: Create job to run `native_unittests` on mandrel builds

[zakkak]

Follow up to oracle#6746

[zakkak]

Note that in Mandrel this is not as trivial as:

mx --primary-suite-path substratevm gate --strict-mode --tags native_unittests

it might require building/running the tests through mandrel-packaging

[github-actions]

This issue appears to be stale because it has been open 30 days with no activity. This issue will be closed in 7 days unless Stale label is removed, a new comment is made, or not-Stale label is added.